This invention relates to a serial type impact dot matrix printer which can print high density dot patterns such as drawing patterns.
A common example of an impact dot matrix printer of serial type is the so called wire dot type printer in which dot patterns of characters and drawings are printed, via an ink ribbon, by impact printing on a printing sheet. A print head, consisting of an array of dot pins constituted by wires arranged in a vertical direction, is moved along a horizontal direction. Of the dot patterns that are printed, an ordinary character dot pattern is a low density dot pattern in which the number of "off" dots which are not printed is greater than the number of "on" dots which are printed. On the other hand, a dot pattern of a drawing where all the dots in a dot matrix are printed, such as a bar of a bar graph, is a high density dot pattern. When such a high density dot pattern is printed more power is consumed for energizing solenoids for driving dot pins than in the case of printing a low density dot pattern of a character or the like, thus resulting in an overload on the power source circuit.
Heretofore, the increase of power consumption at the time of printing of a high density dot pattern has been prevented either by reducing the printing speed or by driving the dot pin array constituting the print head by multi-step printing. However, in the former method control of the carriage's speed of movement and, more fundamentally, adjustment of the motor control has been problematic because of the considerable change in the print head speed and, subsequently, printing speed between high density dot pattern printing and ordinary low density dot pattern printing.
In the latter method, the dot pin array is divided into, for instance, three sections such that not the whole dot pin array, but only a division thereof is driven in one cycle of the print head, thereby printing a one line dot pattern in three print head cycles. By this method the load on the power source is reduced, but a different problem of heat generation is posed. That is, during printing of a dot pattern representing a horizontal bar of a bar graph, dot pins are energized consecutively, although sectionally, so that at least a section of the print head is always hot.